1. Field of the Invention (Technical Field)
The invention relates to means and methods for monitoring and controlling the electrode gap distance during vacuum arc remelting of metallic alloys, particularly melting processes performed at low electrical current.
2. Background Art
Vacuum arc remelting (VAR) is a process used to control the solidification of segregation-sensitive alloys. The design and techniques of VAR have evolved to appreciable levels, as described in U.S. Pat. No. 4,450,570 to Weingartner et al., and patents referenced therein. Generally described, VAR is a process whereby a cylindrically shaped, alloy electrode is loaded into a water-cooled, copper crucible of a VAR furnace, the furnace is evacuated, and a dc electrical arc is struck between the electrode (cathode) and some start material (e.g., metal chips) at the bottom of the crucible (anode). The arc heats both the start material and the electrode tip, eventually melting both. As the electrode tip is melted away, molten metal drips from it and into the molten pool beneath, and the electrode is shortened. The crucible diameter must be somewhat larger than the electrode diameter. Consequently, the ever-shrinking electrode must be translated downwards toward the anode pool surface to keep constant the mean distance between the electrode tip and the pool. The mean distance from the electrode tip to the anode pool surface is called the electrode gap (g.sub.e).
As the cooling water extracts heat from the crucible wall, the molten metal next to the wall solidifies. At some distance below the molten pool surface, the alloy becomes completely solidified, yielding a fully dense alloy ingot. After a sufficient period of time has elapsed, a steady-state situation evolves, consisting of a "bowl" of molten material situated on top of a fully solidified ingot base. As molten alloy solidifies, the ingot grows. The process, performed properly, produces ingots of high homogeneity.
Presently, VAR is the most commonly used melting process to produce ingots for many wrought alloy applications. VAR is particularly well-suited to melting nickel-based "superalloys" (such as Alloy 718) which contain substantial quantities of reactive elements, because melting is performed in a vacuum and the solidification environment can be controlled to the optimum. Among other things, the following improvements in VAR-produced ingots have been noted: (1) contained gases, especially hydrogen and oxygen, are reduced; (2) the alloy is cleaner (fewer non-metallic inclusions); (3) center porosity and segregation in the ingot are greatly reduced; and (4) mechanical properties of the remelted alloy, such as ductility and fatigue strength, are improved.
An important VAR process control parameter is electrode gap. Excessively wide gaps, especially, can be a cause of poor VAR performance, resulting in ingots of reduced quality. Conventional modern VAR controllers use drip-short frequency (f.sub.DS) to control the width of the electrode gap. According to this method, as molten metal drips from the electrode surface, the hanging drop occasionally comes in contact with the anode pool before breaking away from the electrode. This causes the arc to momentarily "short", giving rise to a characteristic signature in the arc voltage trace. The number of these events that occur per second, f.sub.DS, is a function of g.sub.e, and this frequency data can be used to monitor and control the gap.
The reliability of the drip-short frequency method is substantially reduced, however, at reduced melting rates, due to the infrequency of drip shorts. At certain times during the VAR process, it may be desirable to reduce the melting rate (by reducing the electrode current) in order to enhance ingot quality. For this and other reasons, there is a need in the art for a means and method for controlling electrode gap width under conditions of reduced current.